Many mechanical components such as automobile structural components are manufactured through pressing of a steel sheet or other metal sheets. However, products obtained through ordinary cold press forming have a problem that spring back is likely to occur therein due to an intrinsic stress and dimensional accuracy thereof is not stable. As one method to solve this problem, hot pressing called hot stamping has been drawing attention. This hot stamping is a forming method in which a steel sheet heated to a predetermined temperature in advance is press-formed and is quenched in a press die to be hardened. The use of this method prevents the occurrence of the spring back and enables the manufacture of a formed product high in dimensional accuracy and strength.
To perform the hot stamping, it is necessary to heat a steel sheet in advance up to a temperature range at which its metal structure becomes an austenite single phase. As a heating method, a gas heating furnace or the like is generally used, but the gas heating furnace or the like is poor in productivity because of its low heating efficiency. Therefore, to increase productivity, the facility needs to be made large, leading to increased cost. Therefore, as a heating method that can increase productivity, an electrify heating as disclosed in Patent Literature 1 has been proposed. This electrify heating is a method of heating a metal sheet by a Joule heat by passing a current through the metal sheet by bringing electrodes into contact with both ends of the metal sheet, and it has an advantage of wasting less energy and being capable of quick heating. However, when the shape of the metal sheet is not a square but a profile shape, the current concentrates on a portion with a small sectional area, which has a problem that a desired region cannot be uniformly heated. Incidentally, to uniformly heat a specific portion of the metal sheet, laser heating is conceivable, but this has problems of high facility cost and poor productivity.